feat: add privacy mode, thinking levels, PWA support, and i18n

- Add privacy mode to hide IP and user preferences from AI requests
- Add model thinking levels (low/medium/high) for context analysis depth
- Add PWA support with service worker, manifest, and app icons
- Add SettingsPanel for user preferences (theme, background, language)
- Add i18n translations for en/zh/ja/ko/de/fr
- Add Pinia store for centralized settings management
- Update backend to support user preferences and thinking levels
- Update config to use absolute API URLs

public/icons/icon-192.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="192" height="192" viewBox="0 0 192 192">
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0%" stop-color="#0f172a" />
+      <stop offset="100%" stop-color="#1d4ed8" />
+    </linearGradient>
+  </defs>
+  <rect width="192" height="192" rx="42" fill="url(#bg)" />
+  <rect x="42" y="44" width="108" height="104" rx="14" fill="#f8fafc" />
+  <rect x="57" y="66" width="78" height="8" rx="4" fill="#0f172a" />
+  <rect x="57" y="86" width="64" height="8" rx="4" fill="#334155" />
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+  <rect x="57" y="126" width="60" height="8" rx="4" fill="#94a3b8" />
+</svg>

public/icons/icon-512.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="512" height="512" viewBox="0 0 512 512">
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0%" stop-color="#0f172a" />
+      <stop offset="100%" stop-color="#1d4ed8" />
+    </linearGradient>
+  </defs>
+  <rect width="512" height="512" rx="110" fill="url(#bg)" />
+  <rect x="112" y="116" width="288" height="280" rx="36" fill="#f8fafc" />
+  <rect x="152" y="174" width="208" height="20" rx="10" fill="#0f172a" />
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+  <rect x="152" y="324" width="160" height="20" rx="10" fill="#94a3b8" />
+</svg>

public/icons/icon-maskable.svg

@@ -0,0 +1,14 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="512" height="512" viewBox="0 0 512 512">
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0%" stop-color="#0f172a" />
+      <stop offset="100%" stop-color="#1d4ed8" />
+    </linearGradient>
+  </defs>
+  <rect width="512" height="512" fill="url(#bg)" />
+  <rect x="126" y="132" width="260" height="248" rx="32" fill="#f8fafc" />
+  <rect x="162" y="184" width="188" height="18" rx="9" fill="#0f172a" />
+  <rect x="162" y="228" width="156" height="18" rx="9" fill="#334155" />
+  <rect x="162" y="272" width="116" height="18" rx="9" fill="#64748b" />
+  <rect x="162" y="316" width="144" height="18" rx="9" fill="#94a3b8" />
+</svg>